Photodynamic therapy (“PDT”) is a new modality for the treatment of malignancies, diseased tissue, hyperproliferating tissues, pathogens or unwanted normal tissues. PDT involves a localized or systemic administration of a photosensitizing compound followed by exposure of target tissue to photoactivating light. The photoactivating light excites the photosensitizer which, in turn, interacts with singlet oxygen causing the production of cytotoxic oxygen species. The interaction of the cytotoxic oxygen species with tissues in which the photosensitizer is localized causes a modification of the tissue, resulting in a desired clinical effect. The tissue specificity of the resultant phototoxic damage is determined largely, although not entirely, by the relative concentrations of the photosensitizer in each tissue at the time of its exposure to the photoactivating light.
Following systemic administration, many photosensitizers accumulate to varying degrees within tissues depending on the pharmacokinetic and distribution profile of the photosensitizing compound and the cell types comprising the tissues. The chemical factors that enable certain photosensitizers to accumulate to a greater degree at a target site than other photosensitizers is not well understood. Indeed, the biological factors that result in the preferential uptake of some photosensitizers in certain tissue types compared to other tissue types are not well understood either. It is clear, however, that each photosensitizer has its own distribution and pharmacokinetic properties within different tissues and these properties determine the relative usefulness of the photosensitizer for the desired therapy. Currently, rigorous screening and biological evaluation in appropriate model systems is required to identify suitable photosensitizers that display the characteristics necessary to effect a therapy within the diseased or target tissues.
Porphyrins and azaporphyrins and their metallated derivatives belong to a family of substances that are suitable for PDT. These compounds accumulate in target tissues and absorb light in a range in which living tissue is still fairly permeable, namely between 380-680 nm. Moreover, porphyrins, azaporphyrins and their photoactive metallated derivatives exhibit high yields of the excited triplet state, a long lifetime in this state, and good energy transfer to oxygen with concomittent production of singlet oxygen. Of the porphyrins and their derivatives, several photosensitizers have been developed largely for use in oncological applications, but have also been examined in other disease areas in the PDT field in humans. (WO 92/06097; WO 97/20846; EP 0 811626; U.S. Pat. Nos. 5,633,275, 5,654,423, 5,675,001, 5,703,230, and 5,705,622). Such photosensitizers include Photofrin (U.S. Pat. No. 4,882,234), 5-aminolevulinic acid (protoporphyrin IX precursor), SnET2, Visudyne® (Benzoporphyrin derivative), Antrin®, Optrin® (Lutetium texaphyrin) and mono-aspartyl chlorin e6 (MACE). All of these compounds were designed specifically for the treatment of solid tumors. Specifically, many of these compounds were designed to have large absorptions in the 620-740 nm range so as to optimize the photoactivation of the drug with a wavelength that will penetrate to the greatest depths possible all tissue types. In particular, these drugs were designed to absorb outside of the blood absorption profile, thus ensuring efficient photoactivation in most tissue types.
The excitation light source (usually diode lasers or dye lasers) has historically been matched to the far-red absorption bandwidth of the photosensitizer in order to maximize light penetration through tissues. Indeed, the present inventor believes that all the tetrapyrrolic photosensitizers used have been designed for long wavelength absorption of light (>630 nm) to address this perceived issue. Surprisingly, it has been found that short wavelength photosensitizers (with activation absorptions<600 nm) are capable of delivering effective localized therapy to many disease indications where historically long wavelength photosensitizers (with activation absorptions>600 nm) have shown ineffective clinical outcomes. One such example is in coronary artery disease.
While several of the photosensitizers described above have been used to treat atheromatous plaques and some are able to display some inhibition of intimal hyperplasia in animal models, many if not all have characteristics that will limit the usefulness of these drugs in a clinical setting. One particular concern is the half-life of the photosensitizer. A photosensitizer delivered systemically with a long half-life (CASPC, Photofrin, SnET2) may have phototoxic side effects if exposed to direct light, within days of the procedure.
A second even more pressing concern that has to date escaped many of the investigators testing new photosensitizers in cardiovascular disease is photochemically induced damage to “normal” myocardial tissue surrounding the artery due to non-selective photosensitizer uptake and long depths of light penetration, which activates the photosensitizer in the myocardial tissue. Historically, it has been believed that attenuation of the photosensitizer excitation light by blood would inhibit the use of wavelengths of light shorter than 600 nm in the cardiovascular field. This may have been true several years ago when balloon catheter technology in PDT was not as advanced as it is today. New endovascular light ballon catheters, however, can remove most of the blood from the treatment area. This advance enables the use of short wavelengths of light that historically may have been attenuated by blood.
The use of wavelengths of light lower than 600 nm offers significant advantages in PDT because such wavelengths have penetration characteristics that deliver the PDT effect to the target sites (media and adventicia layers of the vessel) and not to myocardial tissue. Thus, effective therapy can be afforded at the target site, while deeper tissues are shielded from a PDT response by blood absorption within these tissues. Previously reported cardiovascular experiments performed to date on tetrapyrrolic molecules have been done at wavelengths>620 nm. Experiments that we have performed in pig arteries with new photosensitizer candidates at light activation>600 nm have resulted in unacceptable levels of damage to myocardial or cardiac muscle tissue surrounding the treatment area. This has major clinical implications to patients with existing ischemic myocardial or muscle tissue due to poor artery perfusion. Attempts to lower the light dosimetry in order to limit treatments to the target tissue (media/intima) leads to long treatment times and less efficacy. In addition, long treatment times in the artery exposes the patient to additional risks with inflation and deflation of the balloon devices. Importantly, we have demonstrated in pig arteries that effective treatment depths can be obtained with shorter wavelengths of light, with sparing of underlying tissue damage.
Thus, it is believed that, long wavelength absorbing molecules (>600 nm), unless highly selective to target myocardial and intimal tissues (which has not to date been reported with any photosensitizer in cardiovascular tissues), may cause unacceptable normal cardiac tissue damage. Therefore, it would appear that activation of lutetium texaphyrin, BPD-MA, MACE, CASPc, SnET2, and pheophorbide PH-II26 with red light may be of limited use in the treatment of cardiovascular disease as all of these compounds are “red” absorbers by design, in so much as all possess low energy absorbtion peaks at wavelengths>600 nm. It should be noted also that chlorins, phthalocyanines and texaphyrin type photosensitizers in general have little absorption in the 500-600 nm regions, and thus may be suboptimal with regard to light activation at green and yellow wavelengths in cardiovascular tissues. In addition, protoporphyrin IX and photofrin do not display absorption maximas at 532 nm, thus they may be inefficient at absorbing treatment light at this wavelength and have very low molar extinction coefficients at 575 nm (˜7000 cm−1/M−1). Furthermore, because long wavelength photosensitizers by design have red absorption peaks, operating room lighting in an emergency situation may cause serious photosensitivity in light exposed tissues. Attempts to use red light filters on operating room lights results in poor tissue contrast and sub-optimal lighting conditions, making surgical procedures under these conditions extremely difficult, if not impossible. Optical clarity is much better at shorter wavelengths (500-600 nm) where the depth of light peneration is limited to a few mm of tissue penetration during the surgical procedure.
Another significant drawback of the above long wavelength absorbing compounds mentioned is that they are only suitable for therapy; prior or simultaneous MRI-diagnostic monitoring of the success of the therapy is not possible with them, nor is radiodiagnostic imaging. For this purpose, it is necessary to administer another paramagnetic substance, which must have a biodistribution that is as close to that of the therapeutic agent as possible. This requirement often cannot be met.
There have been attempts by groups in the field to provide porphyrin linked MRI or radiodiagnostic compounds. Notable examples include: Hilgar, C. S., et al, U.S. Pat. No. 5,849,259; Niedballa, U., et. al., U.S. Pat. No. 5,275,801; Platzek, J., et. al., U.S. Pat. No. 6,136,841; Niedballa, U., et. al., EP 0355041 A2, A3, and B1; Sakata, I., et. al., U.S. Pat. No. 4,996,312; Sakata, I., et. al., U.S. Pat. No. 4,996,312; and Sakata, I., et. al., U.S. Patent No. EP 0220686. It has been known for some time that porphyrin derivatives selectively accumulate in human and animal tumors (D. Kessel and T.-II. Chu, Cancer Res. 43, pp. 1994-1999, 1983; P. Hambright, Bioinorg. Chem. 5, pp. 87-92,1975; R. Lipson et al., Cancer 20, pp. 2250-2257, 1967; and D. Sanderson et al., Cancer 30, pp. 1368-1372, 1972). First attempts to use this class of compound as a diagnostic agent were also described in the literature (J. Winkelmann et al., Cancer Research 27, pp. 2060-2064, 1967; N. J. Patronas et al, Cancer Treatment Reports 70, pp. 391-395, 1986). However, the compounds so far described are far from being able to satisfactorily meet the desired requirements to be effective PDT, MRI and radiodiagnostic imaging agents.
Substituted hematoporphyrin complex compounds used in diagnosis and treatment are described in patent application EP 0 355 041. While these compounds show a good concentration behavior in various target organs, the described compounds used as NMR diagnostic agents are not satisfacatory because they require a dose necessary for optimal imaging that is too close to the lethal dose. Hematoporphyrin derivatives have the drawback that they can eliminate both pseudobenzylic OH groups in the hydroxyethyl side chains.
Derivatives of the deuteroporphyrin have been proposed (Sakata, et al., U.S. Pat. No. 4,996,312 and EP 0220686) for tumor imaging with radioisotopes, containing as additional complexing groups polyaminopolycarboxylic acids bound to the porphyrin skeleton by ethylene glycol bridges (Photochemistry and Photobiology Vol. 46, pp. 783-788 (1987)). However, such porphyrin esters are not very suitable for parenteral use in patients, especially for NMR or radiodiagnostic diagnosis, since the injection solutions obtained from them can neither be heat-sterilized nor stored for a sufficiently long time.
Other derivatives of deuteroporphyrins have been proposed in Hilgar, C. S., et al. U.S. Pat. No. 5,849,259; Niedballa, U., et. al., U.S. Pat. No. 5,275,801; Platzek, J., et. al. U.S. Pat. No. 6,136,841; and Niedballa, U., et. al., EP 0355041 A2, A3, B1 with striking similarity to overcome certain deficiencies of Sakata's deuteroporphyrins by providing metalloporphyrin amide linkages. However, all of these approaches using deuteroporphyrins are suboptimal with respect to design of short wavelength PDT photosensitizers for use as MRI or radiodiagnostic agents for reasons detailed below.
Sakata's porphyrin-based PDT/MRI/radiodiagnostic compounds are based on a naturally occurring asymmetrical porphyrin ring system shown in FIG. 1. In his synthetic philosophy, Sakata has linked polyfunctional carboxyl groups that are capable of binding radioactive metals or MRI active metals to a) the R1 and R2 positions as shown via ether—alcohol linkages; and b) to positions R4 or R5 via ether linking units. This synthetic approach carries with it significant manufacturing problems. First, the linking of one metal chelating moiety to an asymmetrical porphyrin at R1 or R2, R3 or R4 (where R1 and R2 can be vinyl, ethyl, —CH(O-lower alkanoyl)CH3, —CH(OR)CH3 or —CH(O-loweralklene-OR)CH3,) generates at least two new chemical porphyrinic entities in the synthesis process if R1 and R2 (or R3 and R4) are the same linking moiety. This is outlined in scheme 1. 
Naturally occurring porphyrins like hematoporphyrin or protoporphyrin cannot be chemically modified such that only one position, either R1 or R2, (or R3 or R4) is selectively modified to form a molecule with a single linking unit as the only product. In this instance, two compounds are always formed which must be separated to obtain a pure single molecule with which to link the metal chelating moiety. The separation of the two porphyrins is often difficult (if not impossible) and complicates both the manufacturing process and the cost of the final product. If one chooses not to separate the isomers, the isomeric components will each have their own toxicities, and pharmacokinetic and distribution profiles. If one of the isomers is not optimal therapeutically due to any one of these parameters, then the route to regulatory approval is often more complex, time consuming and costly than pursuing a single defined isomer. A second limiting factor that has been highlighted previously, is the instability of the various linking groups to aqueous hydrolysis, elimination at sterilization temperatures, or prolonged storage in solution. Additionally, the use of diastereotopic mixtures as occurs with —CH(OR)CH3 groups in porphyrins complicates the analysis of the molecules for development.
Niedballa and Platzek's approach also has the same synthetic manufacturing problems as explained for Sakata (except when R1═H), i.e., multiple compounds are produced when a single linking moiety is attached to the molecule. These molecules may offer enhanced stability over Sakata's due to the use of amine linkages. The limitation of R1═H symmetry does not, however, allow for modification of this molecule with other functionality that may enhance localization or uptake in tissues or target organelle, or changes in pharmacokinetic or elimination profiles for singly linked molecules. Compounds with high water solubility are often not taken up efficiently by tumors or cells. The ability to enhance the lipophilicity of the molecule is thus very important.
An additional problem, that has been overlooked by all of the prior workers (Sakata, Niedballa, and Platzek) in the development of short wavelength porphyrin photosensitizers, is the limited absortion profile of the porphyrin ring system metallated tetrapyrroles. In general, metallotetrapyrroles have green and yellow absorptions at about 532 and 575 nm with molar extinction coefficients of between 15,000-20,000 M−1 cm−1. In the field of photodynamic therapy, the depth of light penetration into tissues is a function of the wavelength of the exciting light. The theoretical efficacy of a photosensitizer largely correlates to the molar extinction coefficient of the photosensitizer's absorption peak and its ability to absorb light. This is due primarily to the fact that the ability of a photosensitizer to absorb incidental light is a function of the cross sectional area of the molecule's absorption profile. Hence, photosensitizers with low molar extinction coefficients capture photons less efficiently than molecules with high molar extinction coefficients and are thus less efficient.
Therefore, there remains a need for novel photosensitizers that are easily manufactured, have excellent stability and solubility, and have more favorable wavelength absorption characteristics. There is a further need for photosensitizers that are capable of being modified to contain a wide range of substituents making biological targeting more possible and ultimately enabling control of the properties and uses of the compounds clinically for not only MRI and radiodiagnostic imaging, but also for treatment using photodynamic therapy.
The present inventor has found novel metal-free or metallated functionalized phototherapeutic agents that may be used for imaging (MRI or radiodiagnostic) before or after photodynamic therapy. These novel phototherapeutic agents are based on tetrapyrrolic ring systems such as the porphyrins and azaporphyrins that can be covalently linked by stable linkages to metal complexing agents. These new photosensitizers are useful in short wavelength applications in photodynamic therapy.